1. Field of the Invention
The present invention relates to flat microtip display screens and, more specifically, to so-called cathodoluminescent screens, the anode of which supports luminescent elements separated from one another by isolating areas and likely to be excited by an electron bombardment. This electron bombardment requires for the luminescent elements to be biased and can originate from microtips or from layers with a low extraction potential.
To simplify the present description, only microtip screens will be considered hereafter, but it should be noted that the present invention generally relates to the various above-mentioned types of screens and the like.
2. Discussion of the Related Art
FIG. 1 shows an example of a conventional structure of a flat color microtip screen of the type to which the present invention relates.
Such a microtip screen is essentially formed of a cathode 1 with microtips 2 and of an extraction grid 3 provided with holes 4 corresponding to the locations of the microtips. Cathode 1 is placed opposite to a cathodoluminescent anode 5, a glass substrate 6 of which forms, for example, the screen surface.
The operating principle and a specific embodiment of a microtip screen are described, for example, in U.S. Pat. No. 4,940,916 of the Commissariat à l""Energie Atomique.
Cathode 1 is organized in columns and is formed, on a substrate 10, for example made of glass, of cathode conductors organized in meshes from a conductive layer. Microtips 2 are made on a resistive layer 11 deposited on the cathode conductors and are arranged within meshes defined by the cathode conductors. FIG. 1 partially shows the inside of a mesh, without showing the cathode conductors. Cathode 1 is associated with grid 3 which is organized in lines. Grid 3 is deposited on the cathode plate with an interposed isolating layer 12. The intersection of a line of grid 3 and of a column of cathode 1 defines a pixel.
This device uses the electric field created between cathode 1 and grid 3 to extract electrons from microtips 2. The electrons are then attracted by phosphor elements 7 of anode 5 if they are properly biased. In the case of a color screen such as illustrated in FIG. 1, anode 5 is, for example, provided with alternate strips of phosphor elements 7r, 7g, 7b, each corresponding to a color (Red, Green, Blue). The strips can be separated from one another by an insulator 8. Phosphor elements 7 are deposited on electrodes 9, for example formed of corresponding strips of a conductive layer (transparent if the anode forms the screen surface) such as indium and tin oxide (ITO). The sets of red, green, blue strips are for example alternately biased with respect to cathode 1, so that the electrons extracted from the microtips 2 of a pixel of the cathode/grid are alternately directed to the phosphor elements 7 facing each of the colors. In the case of a monochrome screen, anode 5 supports phosphor elements of the same color organized in a single plane or in two sets of alternate strips biased, in this case, separately.
Other cathode-grid and anode structures such as those described hereabove can be found. For example, the anode phosphor elements may be distributed in elementary patterns corresponding to the size of the screen pixels. The anode may further, while being formed of several sets of strips or of elementary patterns of phosphor elements, not be switched by sets of strips or patterns. All the strips or patterns then are at a same potential, for example, by being supported by a conductive plane. The anode is then said to be xe2x80x9cunswitchedxe2x80x9d by opposition to xe2x80x9cswitchedxe2x80x9d anodes where the colors are alternately biased.
The strips of anode patterns supporting phosphor elements to be excited are biased under a voltage of several hundreds of volts with respect to the cathode. In the case of a screen with a switched anode having several sets of strips, the other strips are at a zero potential. The choice of the values of the biasing potentials is linked to the characteristics of the phosphor elements and of the emitting means on the cathode side.
For an electron emission by the cathode microtips, the cathode must be submitted, with respect to grid 3, to a sufficient potential difference. Conventionally, under a potential difference on the order of 50 V between the cathode and the grid, there is no electron emission, and the maximal emission used corresponds to a potential difference on the order of 80 V. For example, the rows of grid 3 are sequentially biased to a potential on the order of 80 V while the columns of cathode 1 are brought to respective potentials ranging between a maximum emission potential and a no emission potential (for example, respectively 0 and 30 V). The brightness of all the pixels in a line is thus set (per color component, if the anode includes several sets of strips selectively biased color by color). The conventional screen control mode consists of forming several images per second, for example from 50 to 60. A duration of approximately 20 ms is thus available to form each image. This duration is called a frame duration.
A problem which arises in a conventional screen is that ions are present in inter-electrode space 13. Indeed, although the inter-electrode space is designed to be under vacuum, the layers constitutive of the different electrodes as well as the residual gases are likely to generate ions under the effect of the electron bombardment. These positive ions are then attracted by the electrode at the lowest potential.
In normal operation (during display frames or sub-frames), the bombardment of the phosphor elements of the anode by the electrons can result in a production of positive ions. These cations are then accelerated towards the cathode that is at the lowest potential and can damage it physically or chemically.
In some screens, a so-called regeneration addressing mode is provided, which consists of biasing, periodically and outside display periods, the cathode microtips into a state of emission while the anode electrodes are biased to a low potential. An example of a control method of this type is described in European patent application No. 0,747,875 of the applicant.
A problem that is then posed is that the electrons emitted by the cathode fall back on the extraction grid since they are no longer attracted by the anode. This phenomenon goes along with an ionization of the species adsorbed at the grid surface. The cations thus generated are then accelerated towards the anode that is at a zero potential, and contaminate the phosphor elements. This ion bombardment of the anode during regeneration phases causes a different aging of the on and off areas of the screen. Indeed, a drop in the light efficiency of the phosphor elements in the off areas during regeneration phases can be observed.
The present invention aims at overcoming the disadvantages of conventional screens by providing a novel structure of a screen with a protection against the undesirable effects of parasitic ions. The present invention aims, in particular, at protecting the cathode against positive ions present in the inter-electrode space. The cathode indeed is particularly sensitive to chemical or physical contaminations. Such is the case, in particular, for microtip screens.
The present invention also aims at providing a solution which is particularly simple to implement and which requires no modification either of the anode, or of the cathode, or of the extraction grid of a conventional screen.
The present invention further aims at providing a solution which can be implemented without modifying the conventional manufacturing of a cathode-grid and of a microtip screen anode.
According to a first aspect, the present invention aims at providing a solution that requires no modification of the conventional addressing of the screen electrodes and, in particular, of the respective addressing potentials of the anode, of the cathode, and of the extraction grid.
According to a second aspect, the present invention aims at protecting the phosphor elements of the anode against an ion bombardment while keeping the possibility of performing conventional regeneration periods.
To achieve these objects, the present invention provides a flat display screen including a cathode provided with field effect electron emission means, a cathodoluminescent anode placed opposite to the cathode, an extraction grid associated with the cathode, and at least one filtering grid, permeable to electron bombardment and biased to forbid parasitic ions generated on one side of this filtering grid to reach the cathode or the anode located on the other side.
According to an embodiment of the present invention, said filtering grid can be biased to a potential greater than a maximum biasing potential of the anode.
According to an embodiment of the present invention, said filtering grid can be biased to a negative or null potential.
According to an embodiment of the present invention, the biasing potential of said filtering grid is switchable between said negative or null potential, outside of display periods, and said potential greater than the maximum biasing potential of the anode, during display periods.
According to an embodiment of the present invention, said filtering grid is placed closer to the cathode than to the anode.
According to an embodiment of the present invention, the screen includes a first filtering grid biased to a potential greater than the maximum biasing potential of the anode, and a second filtering grid, closer to the anode than the first filtering grid.
According to an embodiment of the present invention, the second filtering grid is biased to a potential smaller than the maximum biasing potential of the anode and, preferably, smaller than the minimum biasing potential of the cathode.
According to an embodiment of the present invention, the biasing potential of the first filtering grid is switchable between a negative or null potential outside of display periods and a potential greater than the maximum biasing potential of the anode during display periods.
According to an embodiment of the present invention, said or at least one of said filtering grids is integrated to the anode or to the cathode.
The present invention also provides a method for controlling a screen which consists of, during regeneration periods interposed between display periods, biasing the anode to a potential greater than the potentials of the extraction grid and of the cathode, and biasing the additional grid to a negative or null potential.